I think the idea that a fly "collapses" underwater is bunk lol. The water lifts the fibers and pulses them as you strip it, or pull it into faster water etc...

these will be fished most likely on still water, lakes and ponds..lots of stripping and pausing, which should make the fly breathe pretty well.

Think about an intruder spey fly. If you wet the thing and hold it up, it looks nothing like it does in the water. In the water it looks closer to the dry version with all the marabou puffed up. There is obviously some amount slimming down that occurs, but its not thin as a pencil.